Everyone's given you really good advice so far, but I thought you might want to hear the perspective of someone who has SPD. :)
I would imagine that it probably helps that I spend most of the louder events wearing a large metal hat stuffed with foam...helmets tend to block out sound. Which makes it difficult enough to understand what people are saying even if you don't have issues filtering sounds through your brain. I think it also has probably helped that as a fighter, I had to get used to the sound of my helm getting hit, which can be very loud, and which I still tend to overprocess if I don't see it coming; nevertheless, you learn to anticipate things (like loud sounds), which, for me at least, makes all the difference in the world.
SCA events are loud, but the ones without tournaments going on aren't so bad at all. There are Academy sorts of events in which the Great Hall may be loud-ish, but you can go to a nice, quiet classroom whenever you want and learn about something fascinating that goes on in the SCA. As for tournament events, I'm not sure how I'd be able to deal with them if I weren't fighting. The outside ones are usually better because you don't get the sounds echoing through a giant hall, and often there will be an indoor part of the event for classes or merchants or the feast that you can sit in if it gets too loud outside. But I think it helps me knowing what's going on and being interested in watching because the sounds mean something specific and I can see where they're coming from. I'm not sure how old your son is, but if you think he'd be up to it, you might see if he wants to try youth fighting.
Let's see, what have I forgotten?.... Court is interesting but can get boring if it goes on too long or if you don't know anyone getting awards, so people tend to start getting fidgety as time goes on, which can be really distracting visually and aurally. Often, though, there will be a few people who aren't at court, or who are moving in and out making the feast or whatever, and you'd be fine to step out for a bit if it got to be too much. Feasts are loud; I'm not sure how universal this is for SPD, but I at least tend to be able to tune out loud noises when they are coming from everywhere because they just sort of mush into really loud white noise. The problems I run into at feasts are not being able to hear the people across the table over said white noise (which I think is just a problem generally) and sitting next to someone who eats loudly. Since the general noise tends to drown out most things, you pretty much only hear the people next to you, so if the way they eat doesn't bother you, you're mostly fine. And feasts are pretty relaxed, so you can definitely step outside if you need a break, and there won't really be much of anyone wandering around elsewhere making noise.
I hope that helps! Let me know if you have any more questions :)
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